The present invention relates to an amorphous silicon semiconductor and a process for preparing the same.
With respect to an amorphous silicon prepared from silane (SiH.sub.4) by plasma decomposition, since D. E. Carlson et al prepared as solar cell using an amorphous silicon semiconductor in 1976 based on the discovery by W. E. Spear et al in 1976 that the conductivity of the amorphous silicon is greatly varied by doping with PH.sub.3 or B.sub.2 H.sub.6, a solar cell using a thin film amorphous silicon has been of great interest, and various studies for improving the conversion efficiency of the solar cell have been actively pursued.
Hitherto there has been proposed several types of structures such as the Shottky barrier, p-i-n junction, MIS and hetero junction for a photovoltaic element using thin films of the amorphous silicon semiconductor. Among them, the Schottky barrier type, p-i-n junction type and MIS type photovoltaic elements are useful as a solar cell having a high conversion efficiency. For instace, the Schottky barrier type solar cell has a conversion efficiency of 5.5% (D. E. Carlson et al "IEEE Trans. Electron Devices" EP-24, 449(1977)), the MIS type solar cell has a conversion efficiency of 4.8% (J. I. B. Wilson et al "Nature" 272, 152(1978)) and the p-i-n junction type solar cell has a conversion efficiency of 4.5% (Yoshihiro Hamakawa et al "Surface Sci." 86, 486(1979)).
In a process for preparing the photovoltaic element, however, the growth rate of an an i-layer is low, i.e. 1 to 2 A/sec, which prevents production of a low-cost element. For resolving the defect, various attempts have been made by changing reaction conditions. For instance, there is proposed an attempt for increasing growth rate by increasing power in the glow discharge. When the power is increased, however, properties of the prepared thin film amorphous silicon deteriorate because bombardment of the generated plasma increases (J. C. Knights, "Appl. Phys. Lett." 35(3), 244(1979)). There is also proposed an attempt in which pressure in the operation is adjusted or an attempt in which the temperature of the substrate is varied. However, both attempts have defects in that properties, particularly electrical properties, of the prepared amorphous silicon semiconductor become inferior.
An object of the present invention is to provide a process for preparing an amorphous silicon semiconductor at an increased growth rate without impairing the properties of the semiconductor.
Another object of the present invention is to provide an amorphous silicon semiconductor having good electrical and optical properties.